1. Field of the Invention
This invention relates to vehicle suspension systems, and more particularly to a valve for controlling the internal fluid displacement in a shock absorber.
2. Description of the Related Art
Shock absorbers are used in connection with automobile suspension systems and other vehicle suspension systems to absorb unwanted vibrations which occur during locomotion. To absorb this unwanted vibration, shock absorbers are generally connected between the body and the suspension of the automobile. A piston is located within the shock absorber and is connected to the vehicle body through a piston rod. Because the piston is available to limit the flow of damping fluid within the working chamber of the shock absorber when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force which counteracts the vibration which would otherwise be transmitted from the suspension of the automobile to the body.
A conventional shock absorber comprises a pressure tube with a piston therein and a reserve tube surrounding the pressure tube. A piston rod connected to the piston projects from one end of the pressure tube. At the other end of the pressure tube is a valve communicating with the reserve tube. Damping is controlled by orifices in the piston which regulate passage of fluid from one side of the piston to the other. Due to the presence of a piston rod on only one side of the piston, different volumes of hydraulic fluid must be displaced on the compression and rebound strokes. This difference is the rod volume.
The rod volume of hydraulic fluid is pushed out of the pressure tube during the compression stroke through the valve in the base of the shock absorber. The hydraulic fluid is then stored in the reserve tube which surrounds the pressure tube of the shock absorber.
During the rebound stroke, this fluid which was displaced into the reserve tube through the base valve reenters the pressure tube via the same valve. As the piston moves back and forth within the pressure tube, the rod volume of oil is correspondingly pushed into and out of the reserve tube through the base valve. Thus only a portion of the fluid in the reserve tube is effectively utilized. The remainder remains relatively static within the reserve tube. This quick exchange of fluid as well as the friction between the piston and tube wall generates heat which is very undesirable during prolonged operating conditions. Because of the cyclic fluid exchange through the base valve, the generated heat is concentrated near the base of the pressure tube. The generation of this heat decreases the viscosity of the hydraulic fluid and decreases the lifetime of the operability of the shock absorber.
In addition, because conventional shock absorbers use a base valve, the damping forces generated during compression could not be completely controlled by the amount of damping fluid flowing through the piston. Because the damping forces could not be completely controlled by the amount of damping fluid flowing through the piston, the range of damping which adjustable damping suspension systems could provide was often somewhat limited.